Siloxane-based rubbers such as polydimethylsiloxane (PDMS) are important commercial commodities. PDMS rubbers are commonly produced by the polymerization of cyclic siloxanes in the presence of a base catalyst, in bulk, at elevated temperatures followed by curing of the linear prepolymer. Some examples of such cyclic siloxanes are hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and the like.
Initiation of D4 polymerization by a base (B in FIGS. 1 and 2, e.g., —OH, —OR) to the PDMS prepolymer proceeds by the reaction scheme represented in FIG. 1. Propagation is by repetitive ring opening steps in equilibrium with cyclization, as shown in FIG. 2. This process yields a mixture of linear and cyclic products. The cyclics are removed and recycled. Useful articles are prepared from the linear products by crosslinking. Crosslinking to desirable rubbers may proceed by various techniques, i.e., by the use of peroxides, high-energy radiation, hydrosilation, condensation, and the like.